Kelsey, David E.1,3, Spaggiari, Catherine V.1, Wingate, Michael T.D.1, Lu, Yongjun1, Fielding, Imogen O.H.1, Finch, Emily G.1,2,3
1Geological Survey of Western Australia, 100 Plain St, East Perth, WA 6004, Australia, 2University of South Australia, 101 Currie St, Adelaide, SA 5001, Australia, 3MinEx CRC
Crystalline basement lies beneath the southeastern margin of the Canning Basin and immediately west of the exposed Arunta Orogen, although whether that basement is the continuation of the Arunta Orogen is unknown. The major bounding fault of the Canning Basin coincides with the inferred trace of the Lasseter Shear Zone, which truncates dominant east-trending structures of the orogen and potentially also terminates it. The Top Up Rise prospect is located above a distinct northeast-trending gravity anomaly bound by northeast-trending shear zones coincident with the Lasseter Shear Zone. Five co-funded Exploration Incentive Scheme diamond drillcores from Top Up Rise contain partially melted or melt-injected upper-amphibolite to low-granulite facies basement rocks. These are currently the only drillcores that intersect the Canning Basin basement in this region and provide a means to test its tectonic affinity, and the significance of the Lasseter Shear Zone.
Petrography of granitic gneiss and paragneiss indicates distinctly higher metamorphic grade than is observed in exposed rocks of the west Arunta Orogen, consistent with their separation by a significant shear zone. SHRIMP U–Pb zircon geochronology of several granitic gneiss samples has so far revealed a single magmatic protolith crystallization age of c. 1875 Ma, which is distinctly older than known magmatic rocks in the Arunta Orogen. Maximum deposition ages of 1877–1822 Ma for the metasedimentary rocks are similar to or younger than the magmatic protolith ages of the granitic gneiss, suggesting that emplacement of granitic rocks predated deposition of at least some of the sedimentary rocks. Zircon rims in both granitic gneiss and paragneiss samples record high-grade metamorphism at 1622–1604 Ma. Although metamorphism of this age is unknown in the west Arunta Orogen, thermal events of similar age may have occurred in the central Arunta Orogen (Alessio et al., 2020, Lithos, https://doi.org/10.1016/j.lithos.2019.105280), and granitic magmatism occurred at this time in the Haast Bluff Domain of the Warumpi Province (NTGS Special Publication 5, 2013).
The granitic protolith ages of c. 1875 Ma from Top Up Rise are different to those of granitic suites in the Lamboo Province and Granites–Tanami Orogen; however, felsic volcanic rocks of the lower Halls Creek Group have been dated at c. 1880 Ma (Phillips et al., 2016, GSWA Report 164) and c. 1880 Ma granitic rocks are known in the Arnhem Province (NTGS Record 2017-008). In contrast, detrital zircon age components at c. 1875 Ma are widespread and form significant age components in the Lander Rock Formation of the Arunta Orogen, the Marboo Formation and Tickalara Metamorphics of the Halls Creek Orogen, as well as in Top Up Rise paragneisses. Hence, the newly identified granitic basement in the Top Up Rise drillcores may be representative of a major source component that fed detritus into turbidite fan systems that included the Lander Rock Formation. This basement could represent part of the ‘missing’ Arunta basement, which would support interpretations that the Arunta does continue westwards, or it could be part of a previously unrecognized Proterozoic crustal element that underlies the Canning Basin.
The author team is made up of geologists and geochronologists from the GSWA. Emily Finch is a MinEx CRC Embedded Researcher at the GSWA.